Field of the Invention
The invention concerns a method, a magnetic resonance (MR) apparatus, and an electronically readable data storage medium for optimizing a time progression of an MR control sequence.
Description of the Prior Art
In a magnetic resonance apparatus, the body of an examination object, such as a patient, is exposed to a relatively high basis magnetic field, for example of 1.5 or 3 or 7 tesla, using a basic field magnet of an MR Scanner. In addition, gradient pulses are activated by a gradient coil arrangement. Radio-frequency (RF) pulses, for example excitation pulses, are then radiated by an RF antenna by suitable antenna coils, which results in nuclear spins of specific atoms, which are resonantly excited by these RF pulses, being tilted by a defined flip angle relative to the magnetic field lines of the basic magnetic field. During relaxation of the nuclear spins, RF signals called magnetic resonance signals are emitted, which are received by suitable RF antennas and then further processed. The desired image data can ultimately be reconstructed from the raw data acquired in this manner. In particular, raw data are captured in a spatially defined portion of the examination object under investigation, the examination region. The image data visually depict the examination region.
A specific magnetic resonance control sequence (MR control sequence), also known as a pulse sequence, is composed of a succession of RF pulses, for example excitation pulses and refocusing pulses, together with gradient pulses to be emitted in a matching, coordinated manner in different gradient axes along different spatial directions such as MR sequence must therefore be emitted for a particular measurement. Temporally matching read-out windows are set, which specify the periods in which the induced magnetic resonance signals are acquired. Emission of the RF pulses and the gradient pulses requires power, which is supplied to the magnetic resonance scanner and converted into the corresponding pulses by the magnetic resonance scanner. At least certain components of the magnetic resonance scanner are thermally loaded thereby, and may be heated. It is typically necessary to restrict this heating. The heating may be limited by cooling. This is described, for example, in DE 10 2011 083 204 A1 and DE 10 2007 009 204 A1. In addition, MR control sequences are typically designed such that intervals occur between specific RF pulses and/or gradient pulses and/or at the end of the MR control sequence, which intervals are free of RF pulses and/or gradient pulses. In these intervals, the magnetic resonance scanner is free of any additional significant thermal load.
With health services under increasing cost pressure, the requirement for magnetic resonance examinations to take only a short time is of greatly increasing importance. Despite the introduction of techniques for speeding up the process, such as parallel imaging, “compressed sensing” or simultaneous multislice imaging, the acquisition times for many measurements still lie in the range of several minutes, such that any approach to reducing measuring time further—ideally in combination with the above-stated techniques—is of significance. In some cases hardware component limitations, typically power engineering limitations, determine the limits for further reducing acquisition times. If these limits are not utilized or are estimated too conservatively, measurements take an unnecessarily long time.
Conventionally, heuristic reference parameters are specified for describing hardware components. MR control sequences are designed in accordance with these reference parameters. It is possible in this way to describe a limitation of the gradient coil amplifiers in terms of a reference amplitude: so gradient pulses have an amplitude that is less than the reference amplitude, so that the ability to execute an MR control sequence that includes the gradient pulses is typically ensured over the duration of the examination. Such gradient pulses and MR control sequences are typically selected on a conservative basis and utilize the limitation of the gradient coil amplifiers only for a few gradient pulses in the MR control sequence. No consideration is given to short-term execution of gradient pulses with amplitudes above the reference amplitude.
Model-based methods, as described in DE 10 2008 015 261 B4, make far better use of the available hardware components. In model-based methods, the limiting hardware component is described by a model, whereby amplitudes above the reference amplitude can be used for individual gradient pulses and/or individual sequence modules. In addition, intervals for complying with long-term limitations can be automatically determined. It is thus possible in the short term to use gradient pulses with amplitudes above the reference amplitude if sufficiently long intervals are observed thereafter.